Noninvasive magnetic resonance (MR) molecular imaging and targeted drug delivery systems, often referred to as theranostic agents, are being developed to enable improved detection, patient risk stratification, site-specific treatment, and longitudinal monitoring. Liposomes may be used as non-toxic, biodegradable drug delivery vehicles. However, the biological instability and rapid uptake of liposomes by the retculoendothelial system (RES) after injection restricts their exploitation in the delivery of therapeutic molecules. As an alternative, polymer vesicles may be used, but the resultant nanoparticles are generally polydisperse and suffer from poor shelf life. Moreover, the preparation of block copolymer requires precipitating the block copolymers using a poor solvent, which is tedious and time consuming. The drug release profiles and in vivo bio-distributive properties of these nanoparticles are not controlled and not well understood. Despite these recent advances in developing contrast and therapeutic agent delivery vehicles, there still is a need for nanoparticle delivery compositions that exhibit greater stability and more controlled drug release.